Method of manufacturing a resistance welding electrode

ABSTRACT

The invention relates to a method of manufacturing a resistance welding electrode for welding aluminum or aluminum alloys at the amperage range of 8 to 14 kA. In the assembly for the resistance welding electrode having a combination of a copper or a copper alloy material as a matrix and a stainless steel material positioned in the interior of the matrix the areas made of the stainless steel are positioned parallel to each other in the direction of the electric current flowing through the electrode and have an essential equal distance between two adjacent areas of the stainless steel. The areas are bonded together by hot extrusion and the resistance welding electrode is achieved by cutting the hot extruded product into an appropriate length.

This invention relates to a method of manufacturing a resistance welding electrode to be used to resistance weld sheets of aluminum and aluminum alloys. The resistance welding electrode supplies the electric current or energy to cause a molten weld nugget to be created in the sheet aluminum and a resultant resistance weld. The welding electrode is primarily copper with various insert materials in the form of a matrix in the welding contact face.

It is very difficult to resistance weld aluminum or aluminum alloys due to the high electrical conductivity of aluminum. Therefore, the heat applied by the resistance welding electrode to the aluminum sheets dissipates rapidly away from the electrode contact area. As a result, it requires very high amperages to accomplish a resistance weld. In practice, using any of the currently available commercial resistance welding electrodes amperages as high as 30 kA are required. Further, the high amperage and heat causes the aluminum from the sheet being resistance welded to transfer to the electrode. The resulting alloying of aluminum to the standard copper electrode face results in the electrode sticking to the aluminum sheets after an average 50 welds, and cosmetics of the welded area are poor.

The U.S. Pat. No. 5,611,945 relates to a resistance welding electrode, which has an electrode matrix of a copper alloy and a plurality of members disposed in the electrode matrix. The members are made of a material having an electric conductivity different from that of the electrode matrix. The portions of the members to be exposed on a surface of a tip end of the electrode matrix have a total area between 40 and 82 percent of the entire area of the tip end of the electrode matrix. The material having an electric conductivity different from that of the electrode matrix can be a non-metallic material, like carbon, silicon carbide, titanium carbide, titanium nitride, or a metal material, like titanium, iron, nickel, chromium, a nickel-chromium alloy, niobium or molybdenum.

The U.S. Pat. No. 5,844,194 describes a resistance welding electrode and a method of manufacturing the same, which electrode includes an electrode body as an electrode matrix of a copper alloy and a plurality of filamentary members embedded in the electrode matrix parallel to the direction of the applied electrical current (energy). The resistance welding electrode is quite similar to the one of the U.S. Pat. No. 5,611,945. However, in the U.S. Pat. No. 5,844,194 the exposed tip ends of the filamentary members have a total area between 0.5 and 40 percent of the entire area of the tip end of the electrode body.

According to the U.S. Pat. No. 5,844,194 the resistance welding material is manufactured in the following steps: First the electrode matrix is shaped as a cylinder having a diameter greater than the resistance welding electrode. Then a plurality of holes are drilled in the electrode matrix parallel to the direction to the direction of the applied current such that the total cross-sectional area of the holes is between 0.5 and 40 percent of the cross-sectional area of the electrode matrix after the resistance welding electrode is formed. The holes are filled with a powder or rods of the material having the electric conductivity different from that of the electrode matrix to form the electrode blank. The electrode blank is further shaped into a predetermined configuration. In this configuration the powder or the rods form a respective exposed interior and outermost lands on the surface of the tip end of the electrode matrix so that the interior and outermost exposed lands are positioned on the tip end concurrently at selective respective vertices of a plurality of equilateral triangles and at respective vertices of equilateral polygons containing six or more of the equilateral triangles.

The resistance welding electrodes of the referred U.S. Pat. Nos. 5,611,945 and 5,844,194 include rods which have higher resistance than aluminum and focus the energy in the weld zone, and reduce the rate of heat disposition in the sheet away from the electrode contact area. These US patents describe embodiments in which the resistance welding for aluminum is carried out at the amperage level of 10 kA. However, the weld life of these resistance welding electrodes is very short, only 50 to 200 welds per an electrode. The high cost to manufacture the resistance welding electrodes described in the U.S. Pat. Nos. 5,844,194 and 5,611,945 and the short use life makes them commercially uneconomical.

The object of the present invention is to eliminate the drawbacks of the prior art and to achieve a new and improved method of manufacturing a resistance welding electrode which will weld aluminum or aluminum alloys at the amperage range of 8 to 14 kA and with a useful weld life of the electrode of at least 1000 welds per an electrode before redressing is required. The essential features of the present invention are enlisted in the appended claims.

According to the invention the method of manufacturing a resistance welding electrode for welding aluminum or aluminum alloys is based on a combination of copper or copper alloy and stainless steel (as grade 304), or other appropriate materials so that copper or a copper alloy material forms a matrix for the resistance welding electrode and the stainless steel material is positioned in the interior of the matrix. The assembly for the resistance welding electrode is made of a combination of copper or copper alloy and stainless steel materials so that the areas made of the stainless steel are positioned parallel to each other in the direction of the electric current flowing through the electrode and have an essential equal distance between two adjacent areas of the stainless steel. This assembly is then bonded together by hot extrusion. The resulting hot extruded material is further cold drawn to the final dimension and cut or further formed into a resistance welding electrode or into an insert to be inserted into a resistance welding electrode.

In one preferred embodiment of the invention, the copper or the copper alloy material for the resistance welding electrode is in the form of a plurality of discs with holes for the stainless steel material. When preparing an assembly for the resistance welding electrode the discs are advantageously compacted from a powder of dispersion strengthened copper, or other copper alloy powder with holes for the stainless steel material. The stainless steel material is also in a solid state, such as rods. The compacted discs of the copper or copper alloy material are aligned and layered one above another so that the holes in the individual discs form uniform passages through all the discs. The stainless steel material preferably in the form of rods is positioned into the passages. The assembly is then ready for the hot extrusion in the temperature range of 700-880° C. to a dimension near to the final dimension. After the hot extrusion the hot extruded product for the resistance welding electrode is drawn to the final dimension and cut into an appropriate length in order to produce an insert for the resistance welding electrode. The hot extruded product can be round, oval, square, rectangular, or triangular, but preferably the extruded product is in the form of a rod.

In another preferred embodiment of the invention, a base with holes made of a copper or a copper alloy material is manufactured. This base can be for instance a disc compacted from a powder of dispersion strengthened copper with holes for the stainless steel material, or the base can also be manufactured by casting a copper plate with holes. The stainless steel material to be used in the resistance welding electrode can preferably be coated or electroplated and are preinstalled into the respective holes of the base so that the areas of the stainless steel material preferably in the form of rods are positioned parallel to each other and have an essential equal distance between two adjacent the stainless steel rods. The base with the stainless steel material is inserted to a billet mold. The mold is further filled with molten copper or copper alloy material in order to create a cast extrusion billet assembly for the resistance welding electrode. The matrix of the assembly for the resistance welding electrode is thus manufactured by casting. The coating or electroplating of the stainless steel material will enhance bonding with the molten copper. The coating or electroplating material can be for instance copper, chromium or nickel. The assembly for the hot extrusion is advantageously cooled in a predetermined manner. The assembly is further processed by hot extrusion in the temperature range of 700-800° C. to a dimension near to the final dimension. After the hot extrusion the resistance welding electrode is drawn to the final dimension and cut into an appropriate length.

In one further embodiment of the invention, the assembly for the resistance welding electrode is manufactured so that the extrusion billet exterior is a copper tube and a tube having a smaller diameter made of stainless steel is placed on the interior of the copper exterior tube. The interior tube can also be made from materials other than stainless steel as appropriate. The interior material in a form of a tube or a rod is installed inside the copper tube in the desired positions to the copper tube. Molten copper to be used as the matrix is poured into the copper and stainless steel areas. Alternatively, the copper for the matrix is first cast into a billet made of a copper tube so that during the cast of copper holes for the stainless steel material are created and the stainless steel material in a form of a tube or a rod is then cast into the holes of the billet in a static casting mode or in a vertical down casting billet apparatus. The assembly achieved is then hot extruded in order to bond the matrix material and the stainless steel material together and to get the dimension of the assembly near to the final dimension. After the hot extrusion the hot extruded product for the resistance welding electrode is cold drawn to the final dimension and cut into an appropriate length in order to produce an insert for the resistance welding electrode.

The extruded product is then formed into a resistance welding electrode. After forming, a short length (parallel to the current flow) of the exterior copper can be machined away exposing the stainless steel tube, which is surrounding the interior copper alloy material. During the resistance welding operation, the stainless steel tube focuses the applied current into the aluminum sheet being welded and also restricts the heat flow away from the electrode face thereby allowing a low amperage weld to be achieved.

The stainless steel material (as grade 304) in the method of the invention can then be in the form of a tube or a rod, which can be electroplated or coated in order to enhance bonding with the molten copper. When it is said that the stainless steel material is in a form of a tube or a rod, the tube or the rod can be round, oval, square, rectangular or triangular.

The copper or copper alloy material to be used in the method of the invention is preferably zirconium copper, chromium copper, zirconium chromium copper, beryllium copper or the like as well as copper metal, preferably in the form of a dispersion strengthened copper.

The invention is described in more details referring to the appended drawings wherein

FIGS. 1 a and 1 b show a preferred embodiment of the invention in a schematical manner from a side cross-section view,

FIGS. 2 a and 2 b show another preferred embodiment of the invention in a schematical manner from a side cross-section view,

FIGS. 3 a and 3 b show still another preferred embodiment of the invention in a schematical manner from a side cross-section view.

According to FIG. 1 a and FIG. 1 b the copper or the copper alloy material for the resistance welding electrode is made of a plurality of discs 1 which are compacted from a powder of dispersion strengthened copper. The discs 1 are provided with holes 2 for the stainless steel material so that the holes 2 have an essentially equal distance between two adjacent holes 2. For the resistance electrode the discs 1 are layered one above another so that the holes 2 form uniform passages through all the discs 1. The stainless steel material in a form of rods 3 is positioned into passages through the discs. This assembly containing discs 1 made of dispersion strengthened copper and rods 3 of stainless steel is hot extruded in the temperature range of 700-800° C. to a rod having a dimension near the final dimension in the resistance welding electrode. After hot extrusion drawing to the final dimension is carried out and the rod with the final dimension is then cut into an appropriate length in order to produce an insert for the resistance welding electrode.

FIG. 2 illustrates another preferred embodiment of the invention, wherein the assembly for the resistance welding electrode is made using a mold 11. The mold 11 contains a base 12 made of a zirconium copper by casting. The base is provided with holes 13 so that the holes 13 have an essentially equal distance between two adjacent holes 13. The stainless steel in a form of rods 14 is installed into the holes so that the stainless steel rods 14 are positioned in vertical and essentially parallel positions to each other. Further, the base 12 with the stainless steel rods 14 is inserted into a billet mold 15. Thus the base 12 and the billet mold 15 form the mold 11. The mold 11 is then filled with the molten matrix material 16, molten zirconium copper, i.e. with the same material whereof the base 12 is made. The matrix of the assembly for the resistance welding electrode is made by casting. In order to improve the bonding the stainless steel and the zirconium copper the stainless steel rods 14 are electroplated by nickel before installing into holes 13 of the base 12. Because the base 12 and the molten metal are made of the same material the base 12 is included in the cooled assembly for the hot extrusion. The further process steps to produce an insert for the resistance welding electrode are similar to the ones described for the embodiment of FIG. 1.

In the embodiment of FIG. 3 an extrusion billet 21 for the assembly for the resistance welding electrode has an exterior tube 22 made of copper and another tube 23 made of stainless steel is placed on the interior of the exterior tube 22. The interior tube 23 is installed inside the exterior tube 22. Molten copper 24 for the matrix is cast into the billet 21. The assembly produced is then hot extruded in order to bond the matrix material 24 and the stainless steel material 23 together. Alternatively according to this embodiment, the tube 23 is made of a central core material. When molten copper 24 for the matrix is cast into the billet 24, the central core material is burnt or in some way gone out and holes are formed into the matrix cooled. The stainless steel material is then cast into the holes of the billet in a vertical down casting machine. The assembly is further extruded in order to bond the matrix material 24 and the stainless steel material 23 together. In both cases, the hot extruded product is processed to an insert for the resistance welding electrode in a similar way as in the embodiments in FIGS. 1 and 2. 

1. Method of manufacturing a resistance welding electrode for welding aluminum or aluminum alloys at the amperage range of 8 to 14 kA wherein the assembly for the resistance welding electrode, having a combination of a copper or a copper alloy material as a matrix and a stainless steel material positioned in the interior of the matrix so that the areas made of the stainless steel are positioned parallel to each other in the direction of the electric current flowing through the electrode and have an essential equal distance between two adjacent areas of the stainless steel, is bonded together by hot extrusion and the resistance welding electrode is achieved by cutting the hot extruded product into an appropriate length.
 2. Method of the claim 1, wherein the copper or copper alloy material for the assembly is in the form of discs layered or stacked one above another having holes for the stainless steel material.
 3. Method of the claim 2, wherein the discs are compacted from a non-ferrous powder metal.
 4. Method of the claim 1, wherein the copper or copper alloy material for the assembly is cast into a mold having the preinstalled stainless steel material.
 5. Method of the claim 1, wherein the stainless steel material for the assembly is cast into holes in the matrix material.
 6. Method of the claim 1, wherein the stainless steel material for the assembly is in the form of a rod.
 7. Method of the claim 1, wherein the stainless steel material for the assembly is in the form of a tube.
 8. Method of the claim 1, wherein the stainless steel material for the assembly is electroplated.
 9. Method of the claim 1, wherein the stainless steel material for the assembly is coated.
 10. Method of the claim 1, wherein the assembly for the resistance welding electrode is hot extruded in the temperature range of 700-800° C. 